Automatic altitude control device



L.. MAY ATH TMATIC ALTITUDE CONTROL DE'/IIGE File Auge v5,

INVENmR.

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Patented Aug. 6, 1946 UNITED STATES PATENT OFFICE 14 Claims.

amended April 30, 1928; 370

for the purpose of maintaining flight at a predetermined level.

Present methods of automatic altitude control in radio controlled aircraft contain a number of drawbacks, one of which is that the systems in use are complicated and expensive, which makes the cost factor considerable where such systems are installed in aircraft used for target purposes. Another drawback is found in some systems in that extreme angles of climb'and dive are met.

Such eXtreme angles sometimes cause an aircraft to stall in a down draft greater than the maximum climb rate of the aircraft, or cause the aircraft to dive to a destructive speed'in a violent up draft. Various angle limiting systems have been proposed, one of which is to limit the actuation of the aerodynamic controls of the plane by limiting the extent of rotation of a trim motor adapted to actuate such elements under automatic control responsive to altitude changes. Such methods of limitation, however, are not desirable in radio controlled aircraft, since the trim motor must be capable of obtaining effects of stalling the aircraft to make landings and must also be capable of effecting diving at steep angles to perform various missions. In order to achieve these effects, other systems introduce further complications in the use of trim motors with double limits, or two trim motors, one for automatic altitude control and the other for radio control.

It is an important object of the present invention to provide a simple device for maintaining a predetermined level flight of an aircraft and wherein automatic altitude control means are incorporated comprising elements adapted to limit the extent of automatic control insofar as the rate of dive or climb are concerned.

It is a further object of the invention to provide an automatic control device having features whereby control from the ground by means of radio may be effected independently of the operation of the automatic altitude control system.

In accordance with the above objects, I provide barometric pressure responsive means comprising a lost motion linkage in combination with certain electrical elements disposed for displacement relative each other responsive to changes in altitude of an associated aircraft and wherein an electrical contact engagement occurs between an element displaced in response to barometric pressure and one or the other of a pair of contact portions of another element, depending upon Y whether the aircraft is rising or falling. Contact engagement of the elements is provided periodically by mechanical pressure eans so that the first-named element may be constructed of light flexible material and easily displaced by the barometric pressure responsive mechanism. The contact engagement with one or the other of the contact portions serves to energize an electric motorin one direction or the other, respectively, which actuates aerodynamic control elements of the aircraft, in a well understood manner, and simultaneously displaces the contact portion carrying element in the direction of displacement of the barometric pressure responsive element to restore the initial conditions corresponding to a setting for predetermined level flight. Means arev incorporated in the device whereby the displacement of the barometric pressure responsive element is limited to avoid eXtreme angles of climb or dive. Additional means are provided for actuating the device independently of the automatic control mechanism in order to provide a predetermined flight level by radio control, and by virtue of a lost motion linkage provided in the automatic control system, it is possible to provide independent control by radio for climbing or diving the aircraft at extreme angles under guidance of an operator when so desired.

Other objects and features of my invention will be made apparent by the detailed description which now follows with reference to the appended drawing, in which:

Fig. 1 is a perspective showing the arrangements of the elements thereof; and

Fig. 2 discloses a modification of one of thev elements.

With reference to Fig. 1, the device comprises a contact arm generally indicated as I supported by and keyed to a shaft 3 and disposed for pivotal movement relative a contact or pick-oil plate generally indicated as 5, plate 5 being keyed to a supporting shaft 6. Arm I is pivotally actuated by contraction or expansion of a diaphragm 1 of the type used in conventional altimeters, being responsive to barometric pressure. VDiaphragm 1 is coupled to arm I through a bidirectional lost motion mechanism comprising a slot and pin arrangement I0 and a lever and pin arrangement I3; comprising, respectively, a slotted mem- 3 ber @a secured to the movable end of diaphragm l., a pin lilo, and a lever lc, and, a lever |311, pin |319 and gear sector ic. A gear IB, through which axial shaft 3 passes rotatively, serves to carry diaphragm 'l and the lost motion system by virtue of an integral beam securing the relatively stationary end of diaphragm .'l, and the shaft 2l rotatably supported in any suitable manner in a web of gear it, and having keyed thereto the levers |90 and i3d and rotatably supporting gear sector lc, it being noted that gear sector 3c is thereby provided with vpivotal freedom relav tive shaft 2|. Pin lill? is biased toward engagement with the upper end of the slot in Amember' ma by virtue of a spring 25 secured in tension between beam il and a lever 2e keyed to shaft 2|. lever i3d by virtue of the tendency to uncurl of a spring 39 secured between a post 3| integral with gear i6 and a shafti33. Shaft 33 is coupled at one end by a pinion 35 to gear sector |3c and is rotatably secured at kits other end, in any suitablemanner, to a web of gear i6. The mechanical system thus far disclosed is completed by a gear S8 keyed to shaft 33 and engaging a pinion 39 keyed to shaft 3, whereby reversible pivotal moti-on of contact arm i 4is providedA responsive to functioning of diaphragm l. 'Means for actuating arm l independently of diaphragm 'l is pro* videdin the form of an electric motor llt having a lpinion 4| engaging gear I6, motor 4|! being energizable by a radio contr-ol system, in a wellknown manner, as indicated on the drawing.

,Attention is now invited rto the pick-off system ycomprising arm and pick-off plate 5, as heretofore' described. ,The arm comprises a metallic flexible strip or tongue la secured by an insulating joint vib to a supporting member |c keyed to the shaft 3. The pick-oif plate comprises 'a disc of hardrubber, or the like, having a peripheral ridge supporting ush with the surface thereof a pair of metallic contact segments or. portionsfa and 5b. An insulating portion lc of widthv greater than that of tongue la is provided separating contacts 5a and 5b and is arranged flush with the surfaces thereof. Tongue |a is arranged in spaced relation to the contact portions of plate 5 by a small fraction of an inch, land is brought into periodic engagement therewith by a continuously rotating motor |85 which v'causes reciprocation of a rod 41 lby virtue of a camfmechanism 48, in a well-known manner, thereby oscillating a bar 5d, pivoted at 5|, through force'transmitted thereagainst by a spring 52, rod ll'tbeing slidable in a bore provided in bar 5G.' Oscillation of bar 5S ei'ects bending or deflection of tonguev |a against plate 5 by Virtue of an abutting yoke member 54 having stops 5M and 5th thereon to limit pivotal motion of tongue la. ln' 'practice it 'has been found feasible to space stops 'ia and b apart a distance corresponding' to an altitudevchange of 100 feet. Yoke 54 is secured throughl an insulating member 51 to bar 56.1 A periodic engagement between tongue la and pick-off plate 5 of 2,00 times per minute has been lfound suitable for practical operation. Rod 4T is provided with an end abutment 59 and 'spring' 52 is sufficiently strong to hold bar 50 against abutment 5e until tongue Ia has been pressed firmly against pick-,off plate 5 at some point on the surfaces 5a, 5b or 5c. The stroke of'bar 5i) is such that periodically yoke 54 will clear tongue iatoallow freedom for pivotal movement thereof responsive to functioning of diaphragm l'. During the momentary clamping ac- Pin |319 is biased toward engagement with 4 tion of tongue la against plate 5, no angular displacement of the tongue la is intended to occur even though diaphragm may be expanding or contracting at the time, since the lost motion provided between pin lob and the slot in member la will permit diaphragm 'l to expand without forcing angular displacementv of arm l clockwise, and the lost motion provided between lever 3a and pin |31) will permit diaphragm 1 to contract without forcing angular displacement of arm counterclockwise. The bidirectional lost motion structure is an important feature of the device for the above reason, and for other reasons which will be understood from the description of the manner in which the device operates hereinafter set forth. If it is desired, any suitable length adjusting means 6| may be provided on rod 41 to provide Variation of the time of engagement between tongue la and pick-off plate 5. An electrical ground connection, as indicated on the drawing, is provided for yoke 54 and contacts 5a and 5b are electrically connected through relays 64a and b, respectively, and thence through'battery 66 to ground so that a series connection is established at such times as tongue la engages contact 5a or 5b, thereby energizing the correspondingly lettered relay to close a circuit energizing a motor lil. Motor 1.0 is a reversible type and will be understood to include a reduction gear system having an output shaft 'ma connected in such a manner that the direction of rotation is dependent on whether relay 64a or 64b has been energized .subsequent to Contact of tongue |a with contact 5a or 5b. A linkage system, generally indicated by l2, couples the shaft of motor lll to shaft and a linkage system, generally indicated by 14, couples the shaft of motor l!! to the controls of anassociated aircraft, e. g., the throttle lever, pitch control mechanism, or gyro base line. Motor 'Illl is independently controllable by a radio control system indicated on the drawing, which, of course, may be the sameradio control system indicated in conjunction with the motor 40.. A radio controlled switch is provided at 1| in the circuit of battery 66 for cutting out the automatic' control system at such times as motor 'l is independently operated, the switch at 'Il being cut in by radio control when resuming automatic operation.

In operation, assuming the device is installed on a grounded aircraft and motor 45 is rotating, tongue la is so positioned relative pick-off plate 5 that periodic engagement with insulating surface 5c is established. This'is regarded as normal periodic engagement, motor 'i being deenergized since the surface 5c is a nonoperative point insofar as energization of relays 64a and 64b are concerned. At such time, tongue la is midway between stops 54a and 54o and pin Ib is in abutment with thev top of the slot in member Illa, while lever |3a is in abutment with pin |3b. The device'may then be set for predetermined flight level by energization of motor 4E, through the radio control system or by direct control, whereby gear I6 is rotated counterclockwise by pinion 4| the entire mechanism '|-39 rotating bodily with gear |95, including shaft 3 and arm l. After gear IG is rotated approximately 30 tongue la abuts stop 54a. Continued rotation of gear I6 then serves to wind up spring 30 secured to shaft 33 due to a counterclockwise rotation of gear 38 causedby reaction against the now stationary pinion 39, and simultaneously gear sector |30 is rotated clockwise due to engagement with pinion 35, which rotates with gear 3a. lThe dashed arrows on the drawing indicate the relative rotation of the gear train as just described. Rotation of gear sector I 3c displaces pin |3b away from lever l 3a, thus providing a lost motion gap therebetween. The extent of the lost motion gap determines the height at which the aircraft will level off in night, and is determined in any suitable manner, such as by timing the duration of .energization of motor 40, in relation to a known maximum extent of gap corresponding to the flight ceiling of the particular aircraft. Simultaneously with the foregoing functioning of the mechanism 1-39, periodic engagement between tongue la and contact 5a is occurring, whereby motor I is correspondingly periodically energized by operation of relay 64a, shaft 10a rotating in a counterclockwise direction, thereby setting the control elements of the aircraft for climb, by actuation of linkage 1.4, and simultaneously linkage 12 rotates pick-off plate 5 to restore the condition of normal periodic engagement, i. e., plate follows arm l counterclockwise at a rate determined by the R. P. M. of the reduction gear output shaft of motor 10. Ultimately the insulated surface 5c arrives in position to be periodically engaged by tongue l a which is in its limit position in abutmentagainst stop 54a, thereby breaking the circuit to relay 64a and deenergizing motor 10. Motor 40, however, runs pursuant to operator control and may be stopped either before or after the insulating surface 5c has rotated into register with arm l, the point of stopping of motor 40 being dependent only on the selected flight level.

It will be appreciated that the rate of climb is determined by the arc of travel of arm l prior to abutting the stop 54a, since motor 'l which sets the controls is energized only for the length of time required to effect the followup rotation of plate 5. The followup arc is, of course, identical substantially with the travel arc of arm I, and accordingly limited as a design matter by the arcuate spacing of stop 54a (or 54h in the reverse direction) from the center of yoke 54. It Will be further appreciated that the width of insulating surface 54e relative the width of tongue la determines the extent to which the arm travel arc and the plate followup arc are identical, such relative width being likewise a matter of design which may be varied to suit desired operation, such width likewise determining the extent of altitude variation operationally encountered prior to initiation of corrective response.

The flight level having been predetermined,

assuming the aircraft now takes off, it rises by virtue of the setting of the control elements for climb, as just described. The aircraft continues to rise until diaphragm 1 expands sufficiently to permit rotation of lever Hic, lever 26 and shaft 2| (in a clockwise direction) by virtue of the biasing action of spring 25, to a degree sufficient to close the gap between lever 13a and pin i317.

At this time the aircraft has arrived at the predetermined fiight level but is continuing to rise since the aircraft control velements are still set for climb, whereupon diaphragm l expands further, thus permitting lever 13a to drive gear sector I3c clockwise due to the biasing influence of spring 25, this motion being transmitted through pinion 35, shaft 33, gear 38, pinion 39, shaft 3 to arm l, causing clockwise rotation thereof away from stop 54a. The rotation of arm l .serves to remove the point of periodic engagement from the insulating surface 5c to some point on the contact 5b, the extent of arcuate travel of arm l depending upon the altitude deviation encountered. Relay 64b is thus energized and motor 10 responds accordingly, operating the aircraft control elements to effect diving of the aircraft. Simultaneously, plate 5 is rotated clockwise to followup arm l, which in turn, is now being affected by contraction of diaphragm 1 through the mechanism '1 39 responsive to the present diving of the aircraft so that the rotational direction is reversed, ire., arm l starts moving towards surface 5c while surface 5c is still moving clockwise to follow up the original displacement of arm l, since tongue la is still in periodic engagement with surface 5b. The rate of restoration of normal periodic engagement between tongue la and surface 5c is thus increased, the relative angular velocities of arm I and plate 5 being dependent on the sensitivity of system 1 39, speed of motor 1D and rate of dive, in general. Ultimately, normal periodic engagement is restored as a result of the action just described, and if at the time of restoration lever I3a just abuts pin I3b, the aircraft is operating at the predetermined flight level. If, however, due to conditions of design and/or the existing air currents the aircraft has dived below the predetermined level in the action just described, then diaphragm 'I contracts and causes counterclockwise rotation vof arm l, whereby periodic engagement with surface 5a is resumed at some point corresponding to a materially lessened travel arc of arm I, as compared with the prior engagement therewith, and once more the aircraft climbs. The hunting action thus continues, each reversal having a materially lessened arc of. arm l until equilibrium is obtained, i. e., lever l3nt just abuts pin I3b but transmits no torque to gear sector l3c and arm I is positioned substantially midway between stops 54a and 54h, the tongue la periodically engaging surface 5c, which is the initial `condition of all these elements prior to predetermination of the flight level.

If, for any reason, the aircraft should lose altitude from the predetermined level; diaphragm 1 will contract, thereby actuating the lever |3a counterclockwise, and thus permitting spring 30 to uncurl, the ensuing rotation of shaft 33 driving gear 38 clockwise whence pinion 39, shaft 3 and arm l are rotated counterclockwise. Tongue la is accordingly brought into periodic engagement with the surface 5a, the arc of travel of tongue la being proportional to the rate at which the aircraft is losing altitude as compared with the mechanical inertia of the system |--39, and relay 64a is energized whence motor 'l0 actuates the control elements in the direction of climb. Likewise, if for any reason the aircraft should gain altitude, diaphragm 1 will expand, permitting clockwise rotation of lever i3d under the influence of spring 25, whence the gear train I3c-39 causes clockwise rotation of arm I, whereby engagement of tongue la with surface 5b produces diving actuation of the aircraft control surfaces. In either case, the followup operation of plate 5 is responsive to angular displacement o-f arm l and in the same direction. It will be appreciated that stops 54a and 54h limit the rotary displacement of arm l and therefore limit the angle of climb or dive of the aircraft to avoid unreasonable values which might cause stalling or destructive diving speeds.

With the aircraft in level flight at some predetermined altitude, it is possible to change the altitude to some other predetermined level by remote radio control of motor 4E), the action of the device for predetermining a higher level being the same as that described for preight predetermination. This follows from the fact that the relative positions of lever 13a, pin l3b, arm- I, plate 5 and yoke 54 are restored after the aircraft has found the originally predetermined level, as noted above.

In order to predetermine a level lower than that at which the aircraft is flying, motor 4U is controlled by radio to rotate counterclockwise, whence the system l-SS rotates bodily clockwise until tongue la abuts stop 54h, thus locking pinion 39 against clockwise rotation. Further rotation of motor 40 serves to rotate gear IE, gear 38, shaft 33, and pinion 35 clockwise, causing gear sector |30, pin |3b, lever i3d, shaft 2l and lever Ic to rotate counterclockwise, whence pin lilb rides downwardly in the slot in member lila.

Tongue la having been brought into periodic engagement with surface 5b, the electrical control system becomes operative to dive the aircraft. vThe duration of dive depends on the extent of `lost motion provided between pin lob and the upper end of the coacting slot, which in turn is dependentl on the duration of energization of motor 40, the controllable factor. The dive is 'continued until diaphragm 'l has contracted sufliciently to translate member loa downwardly so that pin Ib abuts the upper end of the slot. Functioning of pick-off plate 5 to followup arm l has been occurring from the time of energization of motor 1U, equilibrium condition being restored after some hunting, due to the aircraft diving beyond the newly predetermined level. Accordingly, with the aircraft in flight, any number of flight levels may be successively predetermined as desired, by radio control of motor lll), due regard being had for the direction of rotation thereof, depending on whether a flight level is desired higher or lower than the existing flight level.

In the event that it is desired to stall the aircraft or to produce a fast or deep dive, motor 'l0 is remotely controlled by radio to operate the aircraft control elements independently of the system I-64b, power from battery 56 being obtained therefor in any suitable manner, such as by providing a multiple switch at il, which automatically disconnects battery 6@ from the system I-Gdb and connects it to motor l in a controllably reversible manner. The details of the radio system and switching arrangement form no part of the present invention and are accordingly indicated only generally on the drawing. By independent operation of motor l0 the aircraft may be brought into extreme stall or dive angles at the will of the operator, and altitude change in either direction, although causing a contraction or expansion of diaphragm T, does not actuate the system i54b to the point of damage since arm l comes to rest against either stop 54a or 54h and contraction or expansion of diaphragm 1 thereafter serves, respectively, to rotate lever i3d away from pin I3b or translate member lila relative pin lb in an upward direction, as seen from the drawing, whereby pin lb rides harmlessly in the slot in member ita.

In Fig- 2 is disclosed a modification of the pickoff plate 5 of Fig. l, wherein the contact portions 18 and 19 differ from contacts 5a and 5b of plate 5 in that they are sloped, being gradiently spaced from the plane of insulating material 80 toward the intermediate insulating portion 82. The sloping construction provides a decreased time of energization of the aircraft control motor, i. e., motor 10, (Fig. 1) would not be energized as much during the complete followup of the pickoff plate shown in Fig. 2 were this plate substituted for the plate 5 of Fig. 1. This follows from consideration of the coaction between tongue la and either of the sloping surfaces 18 andI 19. Where coaction of tongue la is provided with a surface uniformly receding from engagement due to a followup rotation, as is the case of a plate formed as shown in Fig. 2, the duration of each engagement will progressively diminish from the beginning to the end of a followup rotation since progressively more time is used in nonengaging deflection during each deflection cycle of tongue la. Accordingly, motor 10 receives energy impulses of progressively diminishing duration and the aircraft control elements are operated correspondingly. Such a feature is useful in high speed aircraft where response to the aircraft control elements may be disproportionately large as compared with the rate of operation of the automatic altitude control mechanism. By providing a pick-off plate shaped as in Fig. 2, the response of the aircraft will be made to progressively decrease, thereby attaining the advantage of a relatibely large and prompt initial response combined with a gradual tapering off in rate of response so as to make the overall altitude change proportional to the extent of altitude deviation affecting the automatic mechanism rather than to the speed of the aircraft.

Having thus described my invention, I claim:

l. In a device as set forth in claim l2, wherein said altitude response means comprises a gear axially aligned with said contact arm, a pressure responsive diaphragm carried by said gear, a pinion coaxial with said gear and secured for rotation with said contact arm, a driver for said pinion mounted for rotation on said gear, and linkage means connecting said diaphragm with said driver whereby pulsating of said diaphragm responsive to altitude deviations of said associated aircraft is operative to rotate said contact arm in a direction depending on the direction of pulsation of said diaphragm, and means operatively connected to said gear and operable independently of said diaphragm to rotate said gear for the purpose of displacing said contact arm by torque acting on said pinion, and transmitted through said gear, said linkage system and said driver, to predetermine flight level of said associated aircraft.

2. In a device as set forth in claim l2 wherein said means for causing periodic engagement of said arm with said contact member comprises an oscillating member disposed for periodic abutment against said arm in such a manner as to cause exure thereof against said Contact member, said oscillating member carrying stop means thereon whereby displacement of said contact arm responsive to altitude changes of said associated aircraft is limited in response thereto.

3. In a device as set forth in claim 12, wherein said altitude response means comprises diaphragm means responsive to barometric pressure, bidirectional lost motion linkage means connecting said diaphragm means to said contact arm whereby reversible angular displacement thereof corresponding to expansion or contraction of said diaphragm means is realized, said means for causing periodic engagement comprising an oscillating member adapted to abut said contact arm in such a manner as to cause flexure thereof against said contact member, said oscillating member comprising stop means adapted to limit angular displacement of said contact arm in either direction, whereby lost motion in said lost motion linkage means is operative to prevent bending strain'cn said contact arm when said contact arm has traveledto one or the other limits of its angular displacement.

4. In a device as set forth in claim 12, comprising means for rotatably mounting said contact member, electrical relay means connected to said Y contact portions and operative to energize an electric motor in one direction of rotation or the other dependent on periodic engagement of said Contact arm with one or the other of said contact portions, means actuated upon energization of 7 said motor for operating ascent or descent controls of said associated aircraft dependent on the direction of rotation of said motor, and means operatively connected to said contact member and actuated by said motor for simultaneously rotating said contact member in the same direction as the direction of displacement of said contact arm.

5. An automatic altitude control device, comprising a contact arm and a contact plate, said arm being engageable with said plate, said plate having a nonoperative point for engagement with said arm and an operative point for engagement therewith, said arm being adapted normally for engagement with said non operative point, means including a barometric pressure responsive element for displacing said arm relative said plate to effect engagement of said arm with said operative point in response to an altitude change of an associated aircraft, means operative in response to engagement of said arm with said operative point to actuate controls of said aircraft for eifecting an altitude change, and means operative in response to actuation of said lastnamed means to effect displacement of said plate to restore said normal engagement.

6. In a device as set forth in claim 5, including a second operative point on said plate, said nonoperative point being disposed intermediate said operative points, said means for displacing said arm in response to altitude change being operative to effect said displacement reversibly corresponding to gain or loss of altitude, said plate being correspondingly reversbly displaceable to restore said normal engagement.

'7. In a device as set forth in claim 5, including means connecting said arm and said barometric pressure responsive el-ement comprising a lost motion linkage, and means for effecting displacement of said arm for engagement with said operative point independently of altitude changes, for predetermining a flight level deviating from the existing level of said aircraft, means for causing a degree of lost motion in said lost motion linkage subsequent to independent displacement of said arm to a limited extent, whereby said barometric pressure element is mechanically unaffected by said independent displacement of said arm, said altitude response element being operative to take up said lost motion proportionally as said aircraft approaches said predetermined level, the degree of lost motion being proportional to the altitude difference between the predetermined flight level and the existing flight level.

8. An automatic altitude control device, comprising a contact arm and a contact plate, said arm being engageable with said plate, said plate having a nonoperative point intermediate a pair of operative points, said arm being adapted normally for engagement with said nonoperative point at an existing flight level, means including a barometric pressure responsive element for displacing said arm relative said plate to effect engagement of said arm with one of said operative points dependent on a gain or loss of altitude of an associated aircraft, means operative in response to engagement of said arm with said operative point to actuate controls vof said aircraft for effecting a compensating altitude change, means operative in response to actuation of said last-named means to elect displacement of said plate to restore said normal engagement, means connecting saidrarm and said barometric pressure responsive element comprising a bidirectional lost motion linkage means, and means for effecting reversible displacement of said arm for engagement with one or the other of said operative points independently of altitude changes, for predeterinining a llight level higher or lower than the existing flight level, means for causing a degree of lost motion in said bidirectional lost motion linkage means, alternatively, 'depending on the direction of independent displacement of said arm subsequent to displacement thereof in either direction to a limited extent, whereby said barometric pressure responsive element is mechanically unaifected by said independent displacement of said arm, said altitude responserelement being operative to take up said lost motion proportionally as said aircraft approaches said predetermined level, the degree of lost motion being proportional to the altitude difference between'the predetermined flight level and the existing flight level.`

Y9. An aircraft control device comprising, in combination, a pair of contact elements, means for causing engagement of said elements, engagement thereof normally occurring at a nonoperative point therebetween, means whereby said elements are displaceable relative each other, relative displacement between said elements being effective to shift the point of engagement to an operative point therebetween, means responsive to engagement at said operative point effective to actuate control means of said aircraft, and means responsive to actuation of said control means to provide an opposite relative displacement between said elements whereby engagement at said inoperative point is restored.

10. In a device as set forth in claim 9, wherein said means whereby said elements are displaceable relative each other includes an altitude responsive element and lost motion linkage means connecting said altitude responsive element to one of said contact elements for displacement thereof responsive to altitude changes, stop means for limiting displacement of said contact element, whereby, when said contact element is at the limit of displacement thus provided, further altitude change is operative to effect lost motion compensation in said lost motion linkage means to relieve additional displacing force on said contact element and whereby the extent of actuation of said aircraft control means is correspondingly limited.

11. In a device as set forth in claim l2, wherein said plate comprises a pair of contact members having surfaces which slope in relation to the plane of displacement thereof, whereby periodic engagement with said arm occurs with progressively varying duration of contact as said plate is displaced relative said arm.

12. In an automatic altitude control device, a contact arm, a contact member having a pair of elongated contact portions separated by an insulated portion, said contact arm being normally positioned to engage said insulated portion only, altitude sensitive means responsive to changes in altitude of an associate aircraft whereby said arm is displaceable away from said insulated portion and along one or the other of said elongated contact portions, means for periodically causing engagement of said contact arm with said contact member, normally against the insulated portion but displacement of said arm along said elongated contact portions in response to altitude changes being operative to change the point of periodic engagement of said arm with said contact member to one of the contact portions thereof, means responsive to contact between said arm and one of said contact portions to vary the controls of said associate aircraft to provide ascent or descent depending on the specific contact portion engaged, and means responsive to periodic engagement of said arm with said specific contact portion to displace said contact member in the direction of the displacement to restore said normal periodic engagement.

13. In a device for bringing an associated aircraft to a preselected altitude and maintaining it at said altitude, a co-ntrol motor operable from a neutral position in one or the other directions to set the controls of the associate aircraft for climb or div-e, a first electric switch element carrying two elongated insulated contacts with an insulating space therebetween, one electrically connected to each motor terminal and arranged to convey current to revolve said motor in one or the other direction, linkage connecting said control motor to said first electric switch element for moving said elognated contacts in coincidence with movement of said controls, a second electric switch element comprising a single contact for conveying current to one or the other of said elongated contacts, normally situated in said insulating space and out of contact with said elongated insulating contacts but movable along said elongated contacts in either direction, independently of the movement of said iirst electric switch element, means to selectively set said second electric switch element in one or the other direction along one or the other of said elongated insulated contacts, whereby said control motor operates said controls for climb or dive and coincidentally moves said insulating space to a position in alignment with said single contact, an altitude sensitive means, and linkage connecting said altitude sensitive means to said second electric switch element operative by movement of said altitude sensitive means upon change in altitude of said associated aircraft to return said single contact to the starting position, whereby said control motor returns to the neutral position for maintaining the selected altitude.

14. The device defined in claim 13 in which the first electric switch element is a rotatable dielectric disc, the elongated insulated contacts being arcuate metal pieces carried in said disc, and the second electric switch element is an independently rotatable conductive contact arm adapted for contact with said arcuate metal pieces.

ROBERT L. MAYRATH. 

